There are two types of paving technologies used today for producing asphalt-based formulations for application to a surface to produce pavement: hot mix paving and cold paving. In hot mix paving, aggregates are heated to a temperature above 200° C. to remove residual water and mixed with molten asphalt at a temperature above 165° C. Because hot mix paving requires providing the aggregate and the asphalt at high temperatures, there are considerable energy requirements and cost associated with hot mix processes.
Cold paving does not involve heating the components used to produce the asphalt-based formulation. The typical cold paving method includes mixing aggregates with Portland cement or lime, wetting the aggregates with water, and adding an asphalt emulsion to the wetted mixture. These components are then vigorously mixed and applied to the road surface. For high traffic areas such as highways and major roads, the asphalt emulsion is typically modified through the use of a polymer such as a styrene-butadiene copolymer to increase the strength and durability of the paved surface. These formulations are commonly known as microsurfacing formulations. For low traffic areas such as small neighborhood roads, slurry seal formulations can be used without the need to modify the asphalt emulsion; however, the use of the polymer may still be desired.
Although there are numerous benefits associated with cold paving, including the reduced cost compared to hot mix paving, there are also problems associated with this method. For example, because Portland cement and lime are used as a fine powder, it is hard to accurately measure the amount needed for use in the cold paving formulation at the mixing truck. In addition, the fine Portland cement and lime powder can blow away when it is being mixed with the aggregate on top of the mixing truck.
Another problem associated with conventional cold paving formulations is that they are very sensitive to the application temperature. For example, in cold weather, the cohesion development of the formulation is slow and, as a result, long curing times exceeding an hour are necessary before the road can be opened to traffic. In hot weather, the formulation has a short mix time as defined by the period of time between the time the components are mixed and the time they must be applied to the road surface. Therefore, the formulation must be applied to the road surface quickly before the viscosity of the formulation increases to the point that it cannot be readily applied. Another problem with hot weather climates is that a formulation may work well in the cooler morning but by afternoon, when the temperature has increased, the asphalt emulsion breaks prematurely resulting in water breaking out of the formulation and accumulating between the old road surface and newly applied surface. This problem is known as the tenderness problem and results in the newly added pavement peeling off the existing surface once the road is opened to traffic.
Cold paving formulations are also very sensitive to the emulsion pH and acidity of the asphalt used in the formulations. Traditionally, an asphalt having a high acid number such as Venezuela asphalt has been used because it is difficult for the newly applied pavement to develop enough cohesion when a low acid number asphalt is used. Furthermore, asphalt emulsions having a low pH of 1.0 to 1.5 have been used because asphalt emulsions having higher pH's either result in a newly applied pavement that does not develop enough cohesion or that has very slow cohesion development. As a result, it takes more than one hour for the road to be available for traffic. Therefore, the types of asphalts that can be used for cold paving have traditionally been limited only to asphalts having high acid numbers and low emulsion pH's. Furthermore, there have been a lot of safety problems associated with low pH asphalt emulsions. For example, low pH asphalt emulsions can corrode or even eat through storage equipment.
Cold paving formulations are also sensitive to the type of aggregate used. In particular, aggregates may have different properties such as different fines contents and may interact with cationic surfactants present in the asphalt emulsion differently. Thus, the asphalt emulsion often must be modified such as by varying the use of certain surfactants to provide useful paving formulations.
There is a need in the art of paving formulations to produce a method and formulation for cold paving that is less sensitive to parameters such as aggregate types, the type of asphalt used to prepare the asphalt emulsion, the type and amount of the surfactants used in the asphalt emulsion, the emulsion pH and the application temperature. In addition, there is a need in the art to overcome the problems associated with the use and metering of Portland cement and lime for use in these formulations. There is also a need to produce a cold paving formulation that has a desirable mix time and good curing behavior.